My daily dumb question is what are micro balloons and do I need them?

On page 23 step 2 of the [link=http://www.greatplanes.com/manuals/gpma0220-manual.pdf]Manual[/link]
It says to mix a small batch of 30 minute epoxy and microballoons.
Use the mixture to glue the guide tubes in the exit slots...

Can I glue the tubes in without micro balloons?